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How to Determine the Chloride Concentration in an Alkali Solution

Like most nitrates, silver nitrate (AgNO3) dissolves well in water. Silver chloride, however, is highly insoluble. That's why you can use the lab reagent silver nitrate to determine chloride concentration -- the silver ions from the silver nitrate will combine with the chloride ions to form solid silver chloride. In basic or alkaline solution, you can use a chemical called potassium chromate as an indicator to figure out when all of the chloride has been precipitated.

Things You'll Need

Gloves, goggles, lab coat
Standardized 0.1 M silver nitrate solution
Buret
Funnel
Ring stand
Clamp
250 mL Erlenmeyer flask
Graduated cylinder
Alkaline solution whose chloride concentration you want to determine
5 percent potassium chromate (K2CrO4) solution
20 mL pipette

Instructions

- 1
  Put on your gloves, goggles and coat.
- 2
  Rinse the buret and fill it up with silver nitrate solution. Using the funnel will help you avoid spills when you add silver nitrate. Do not try to add silver nitrate to the buret above eye level. Make sure that the stopcock on the buret is closed before you begin adding silver nitrate to it.
- 3
  Secure the buret to the ring stand with the clamp.
- 4
  Measure out 50 mL of the alkaline solution whose chloride concentration you want to determine and add it to the Erlenmeyer flask. Add a couple milliliters of the potassium chromate solution and gently swirl.
- 5
  Place the Erlenmeyer flask beneath the tip of the buret.
- 6
  Open the stopcock on the buret and slowly add a few drops at a time of silver nitrate reagent. You will notice that a solid precipitate forms, causing the solution to turn cloudy, and the solution will take on a faint yellow color. Swirl the flask gently as you add more silver nitrate.
- 7
  Cease and desist immediately when a reddish-brown precipitate forms and persists in the solution for at least 30 seconds. It's important not to overshoot and keep on adding silver nitrate even after the formation of the reddish-brown precipitate, because this will give you false results.
- 8
  Look at the buret to see how many milliliters of silver nitrate reagent you added. Take this value and divide it by 1000 to convert it to liters, then multiply it by 0.1 to get the number of moles of silver nitrate you added. This is equal to the number of moles of silver ions you added, which is approximately equal to the number of moles of chloride ion present.
- 9
  Divide the number of moles of chloride ion present by 0.05 L, which was the original volume of the solution you were testing.

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